The tmpfs facility allows the creation of filesystems whose contents
reside in virtual memory. Since the files on such filesystems
typically reside in RAM, file access is extremely fast.
The filesystem is automatically created when mounting a filesystem
with the type tmpfs via a command such as the following:
$ sudo mount -t tmpfs -o size=10M tmpfs /mnt/mytmpfs
A tmpfs filesystem has the following properties:
* The filesystem can employ swap space when physical memory pressure
demands it.
* The filesystem consumes only as much physical memory and swap
space as is required to store the current contents of the filesys‐
tem.
* During a remount operation (mount -o remount), the filesystem size
can be changed (without losing the existing contents of the
filesystem).
If a tmpfs filesystem is unmounted, its contents are discarded
(lost).
Mount options
The tmpfs filesystem supports the following mount options:
size=bytes
Specify an upper limit on the size of the filesystem. The
size is given in bytes, and rounded up to entire pages.
The size may have a k, m, or g suffix for Ki, Mi, Gi (binary
kilo (kibi), binary mega (mebi) and binary giga (gibi)).
The size may also have a % suffix to limit this instance to a
percentage of physical RAM.
The default, when neither size nor nr_blocks is specified, is
size=50%.
nr_blocks=blocks
The same as size, but in blocks of PAGE_CACHE_SIZE.
Blocks may be specified with k, m, or g suffixes like size,
but not a % suffix.
nr_inodes=inodes
The maximum number of inodes for this instance. The default
is half of the number of your physical RAM pages, or (on a
machine with highmem) the number of lowmem RAM pages, which‐
ever is smaller.
Inodes may be specified with k, m, or g suffixes like size,
but not a % suffix.
mode=mode
Set initial permissions of the root directory.
gid=gid (since Linux 2.5.7)
Set the initial group ID of the root directory.
uid=uid (since Linux 2.5.7)
Set the initial user ID of the root directory.
huge=huge_option (since Linux 4.7.0)
Set the huge table memory allocation policy for all files in
this instance (if CONFIG_TRANSPARENT_HUGE_PAGECACHE is
enabled).
The huge_option value is one of the following:
never Do not allocate huge pages. This is the default.
always Attempt to allocate huge pages every time a new page is
needed.
within_size
Only allocate huge page if it will be fully within
i_size. Also respect fadvise(2)/madvise(2) hints
advise Only allocate huge pages if requested with
fadvise(2)/madvise(2).
deny For use in emergencies, to force the huge option off
from all mounts.
force Force the huge option on for all mounts; useful for
testing.
mpol=mpol_option (since Linux 2.6.15)
Set the NUMA memory allocation policy for all files in this
instance (if CONFIG_NUMA is enabled).
The mpol_option value is one of the following:
default
Use the process allocation policy (see
set_mempolicy(2)).
prefer:node
Preferably allocate memory from the given node.
bind:nodelist
Allocate memory only from nodes in nodelist.
interleave
Allocate from each node in turn.
interleave:nodelist
Allocate from each node of in turn.
local Preferably allocate memory from the local node.
In the above, nodelist is a comma-separated list of decimal
numbers and ranges that specify NUMA nodes. A range is a pair
of hyphen-separated decimal numbers, the smallest and largest
node numbers in the range. For example,
mpol=bind:0-3,5,7,9-15.

In order for user-space tools and applications to create tmpfs
filesystems, the kernel must be configured with the CONFIG_TMPFS
option.
The tmpfs filesystem supports extended attributes (see xattr(7)), but
user extended attributes are not permitted.
An internal shared memory filesystem is used for System V shared
memory (shmget(2)) and shared anonymous mappings (mmap(2) with the
MAP_SHARED and MAP_ANONYMOUS flags). This filesystem is available
regardless of whether the kernel was configured with the CONFIG_TMPFS
option.
A tmpfs filesystem mounted at /dev/shm is used for the implementation
of POSIX shared memory (shm_overview(7)) and POSIX semaphores
(sem_overview(7)).
The amount of memory consumed by all tmpfs filesystems is shown in
the Shmem field of /proc/meminfo and in the shared field displayed by
free(1).
The tmpfs facility was formerly called shmfs.

This page is part of release 5.01 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2019-03-06 TMPFS(5)